Recently, various information about movies, music, books, and the like has been digitalized to data, and such digital data has been handled as commercial products. Hereinafter the digitalized data of the various information about movies, music, books, and the like is referred to as “content data”.
The content data, in most cases, is recorded onto an information recording medium, such as a DVD, and is provided from a seller to a buyer through the information recording medium. Moreover, lately, the content data has been provided from the seller to the buyer through a computer network, such as the Internet, more and more.
By the way, the content data is digital data. The digital data does not deteriorate even if it is copied. That is, the quality of video or audio recorded as the digital data does not deteriorate even if it is copied. Thus, in order to protect copyright of the video, music, or the like, some invention for restricting the illegal reproduction, illegal copy, and other illegal handling of the content data is provided for the content data, an information recording medium on which the content data is recorded, or a reproducing apparatus for reproducing the content data. For example, a DVD employs a technology for restricting a reproducing apparatus capable of decoding the content data, which is recorded on the information recording medium after being encrypted, and a technology for restricting the copy of the content data.